The invention relates to a method of and an apparatus for sealing plastic-metal laminates, and more particularly to the hermetic sealing of light, metal containers coated with a plastic material.
The use of lightweight containers, particularly the use of relatively large containers having a capacity of more than 1 liter, is becoming increasingly accepted in the field of preserving food by heat sterilization in metal containers. Containers of this type consist of a lightweight metal foil coated with a heat-sealable plastic material, such as polypropylene or other similar materials. The hermetic sealing of containers of this type is achieved by joining the edge of the lid to the rim of the container in such a way that the two plastic layers are homogenously joined to one another by applying a suitable temperature and a suitable pressure for a suitable length of time.
With heat seals of this type, it is necessary to seal a relatively large surface which has often been contaminated during the filling procedure by the goods to be preserved. Because of the working cycle time, the rim can only be quickly wiped by the operating personnel, so that at least some moisture is still present on the hydrophobic surfaces to be heat-sealed. Heat-sealing can only be made satisfactory if close tolerances of the temperature, pressure and time are maintained. With smaller containers, these parameters are particularly critical. It must be possible to adjust the relevant values as well as to measure them, and the values, once set, must not change since even a very minor defect in the heat-seal seam inevitably results in spoilage of the goods. The number of faulty products may build up to a very considerable extent, with obvious disadvantageous consequences, before such defects are found.
As to the actual sealing pressure and temperature, it is essential in this connection that the values determined to be optimum are continuously maintained, and that in view of the variations within the limits of tolerance of the thickness of the lightweight, metal sheet material and the plastic coating, it must be possible to readjust the values of these parameters. This can be effected by respective control means depending on the determined variations within the range of tolerance.
Control of the temperature does not pose any particular problems since a fine adjustment can be set by appropriately arranged temperature probes. However, the pressure conditions and the time factors, namely, the duration of application of the actual sealing pressure, as well as the length of time which is required for build-up of this heat-sealing pressure and for the subsequent restoration of the normal pressure, are particularly critical. Within a possible range of sealing pressures, which depends on the type of plastic material and on the temperature, the higher the pressure, the shorter is the sealing time and the more effectively will contaminations from the filling procedure, present on the seal surfaces, be forced out of the seal zone. Moreover, the time required for build-up of the full sealing pressure should not be more than 20% of the total sealing time. The sealing time is to be understood as the total interval between the beginning to the end of contact between the upper and the lower sealing dies while the surfaces to be heat-sealed are interposed.
It is known to apply the pressure forces required to meet these demands, by means of fluids or by a cam mechanism.
Pneumatic application of the sealing pressure by means of a gas, such as air, has the disadvantage of a slow build-up of pressure as a result of the compressibility of the gas. Residues of the goods being filled are thus not sufficiently reliably squeezed from the seal surface and may bake onto the plastic, thus making the hermetic sealing impossible.
The sealing time is set by means of a time switch, which in turn controls a valve to reverse the pressure cylinder after the set time. Since devices of this type are always subjected to high, relative, atmospheric humidities, the formation of condensation in the air system cannot be prevented. After a time, switching inaccuracies will occur for this reason, so a reproducible sealing time cannot be guaranteed. Since the pressure cylinder has to be filled twice for each sealing step, a high consumption of air also results and, furthermore, pressure fluctuations in the compressed air system lead to varying sealing pressures.
Hydraulic application of the sealing pressure has substantially the same disadvantages as pneumatic application, with the exception of those which are ascribed to the compressibility and to the formation of condensation. Even here, a relatively slow build-up of pressure results, and it has already been pointed out that this is a critical parameter. Additionally, switching inaccuracies can easily occur in the hydraulic valves.
Application of the sealing pressure by means of a cam mechanism, using cam discs or cam cylinders, will result in a rapid build-up of pressure. However, since the upper and lower sealing dies must be sprung by means of discs, screws or rubber springs, the sealing pressure can be set to a definite required value only with difficulty. After a certain time, the spring elements suffer fatigue, so that the conditions move out of the critical pressure range.
In the art, a quite considerable number of defective seals still occur with preserving containers having capacities over 1 liter, despite the optimization of the duration of sealing pressure and the values of the sealing pressure and the temperature and despite appropriate readjustment of the relevant values. After some time, this manifests itself in so-called "flippers"; that is to say, the seals are not hermetic and result in spoilage of the contents.
A technological solution of such problem has now, in surprising manner, been rendered possible by the finding that there obviously exists a fourth, critical parameter in the sealing process which has not been earlier recognized. This fourth parameter is the time period between the initial application of the pressure until the actual sealing pressure had been established.
Lengthy investigations have shown that the period of pressure build-up to the actual sealing pressure should not be longer than 20% of the period during which the sealing pressure is applied, and preferably should be substantially less. In particular, this time period should be less than 10%, and preferably be within the range of 3%-5%.
Holding the ratio of the duration of pressure build-up to the sealing pressure within the preferred range results in particularly reliable sealing. The percentage of rejects in comparison with a process where the time interval of pressure increase is outside of the inventive range, is lower by at least the order of a power of ten.